Only set contextTask if not adding a new task

When adding a new task, the current task is empty, so there is no
context in which to evaluate DOM references.  #2683 will address this in
a more robust fashion.

src/columns/ColProject.cpp

@@ -121,7 +121,9 @@ void ColumnProject::modify (Task& task, const std::string& value)
     {
       Eval e;
       e.addSource (domSource);
+      if (!task.is_empty ()) {
         contextTask = &task;
+      }
 
       Variant v;
       e.evaluateInfixExpression (value, v);

src/columns/ColRecur.cpp

@@ -108,7 +108,9 @@ void ColumnRecur::modify (Task& task, const std::string& value)
   {
     Eval e;
     e.addSource (domSource);
+    if (!task.is_empty ()) {
       contextTask = &task;
+    }
     e.evaluateInfixExpression (value, evaluatedValue);
   }
 

src/columns/ColTags.cpp

@@ -162,7 +162,9 @@ void ColumnTags::modify (Task& task, const std::string& value)
   {
     Eval e;
     e.addSource (domSource);
+    if (!task.is_empty ()) {
       contextTask = &task;
+    }
 
     Variant v;
     e.evaluateInfixExpression (value, v);

src/columns/ColTypeDate.cpp

@@ -213,7 +213,9 @@ void ColumnTypeDate::modify (Task& task, const std::string& value)
   {
     Eval e;
     e.addSource (domSource);
+    if (!task.is_empty ()) {
       contextTask = &task;
+    }
     e.evaluateInfixExpression (value, evaluatedValue);
   }
 

src/columns/ColTypeDuration.cpp

@@ -55,7 +55,9 @@ void ColumnTypeDuration::modify (Task& task, const std::string& value)
   {
     Eval e;
     e.addSource (domSource);
+    if (!task.is_empty ()) {
       contextTask = &task;
+    }
     e.evaluateInfixExpression (value, evaluatedValue);
   }
 

src/columns/ColTypeNumeric.cpp

@@ -55,7 +55,9 @@ void ColumnTypeNumeric::modify (Task& task, const std::string& value)
   {
     Eval e;
     e.addSource (domSource);
+    if (!task.is_empty ()) {
       contextTask = &task;
+    }
     e.evaluateInfixExpression (value, evaluatedValue);
   }
 

src/columns/ColTypeString.cpp

@@ -67,7 +67,9 @@ void ColumnTypeString::modify (Task& task, const std::string& value)
   {
     Eval e;
     e.addSource (domSource);
+    if (!task.is_empty ()) {
       contextTask = &task;
+    }
 
     Variant v;
     e.evaluateInfixExpression (value, v);